1. Field
The present application is related to analog-to-digital converters and more specifically to multi-mode analog-to-digital converters.
2. Description of the Related Art
Analog-to-digital converters (ADCs) are used widely in signal processing chains ranging from low-frequency applications (e.g., instrumentation, voice, audio, etc.) to relatively high-frequency applications (e.g., DSL, WiFi, TV). Typically, for high-frequency applications such as analog and digital TV (e.g., where channel bandwidth may be between 6 and 8 MHz for terrestrial and cable signals and up to 30 MHz for satellite signals), low oversampling ratio (OSR), higher order delta-sigma modulators with multi-bit quantizers are often used. Some delta-sigma modulators employ distributed feed-forward (DFF) topologies, which may feature reduced internal signal swings and may use fewer feedback digital-to-analog converters (DACs). However, DFF topologies may increase effects of interfering signals, such as by increasing out-of-band signal transfer function (STF) peaking The peaking may be more pronounced in ADCs with multi-bit quantizers because more aggressive loop scaling can be done without compromising loop stability compared to delta-sigma modulators with single-bit quantizers.
Out-of-band STF peaking is a serious concern for applications where large close-in blockers may be present in the vicinity of the desired channel (e.g., television applications). A baseband or intermediate frequency (IF) filter preceding the ADC may not reduce these blockers sufficiently to ensure stability of the ADC. Although the ADC input level may be increased to ensure that residual blockers do not overload the ADC, a reduction in the ADC input level may reduce the in-band dynamic range of the ADC. Further, blockers may appear at varying frequencies, which may make accounting for the blockers a significant design challenge, potentially increasing cost and complexity of the ADC.